A few days ago, I built my own publishing form on my blog to be able to create new posts on the go. It works great, but Micropub would be even better, because I could also use Micropub-compatible apps. Just today I found Indiekit, which is exactly what I am looking for:
The immediate goal of this project is to provide a Micropub endpoint that can be hosted on a service like Heroku, configured via files stored on a GitHub repo, and save posts back to that repo for publishing with a static site generator such as Eleventy, Hugo or Jekyll. The software is fully documented and tested.
I probably just need to find a way to use Gitea instead of Github, but that shouldn't be a big problem. I need to further investigate Indiekit.